MeltDBWiki/RoundingMZ

Round m/z values of mass spectra to integral values

• Several mass spectral databases contain mass spectra in nominal m/z notation whereas MS instruments are able to obtain higher resolution.
• High resolution mass spectra can be compared to nominal m/z spectra by rounding the measured m/z values to integral values.
• The KEGG compounds database contains more than 10000 compounds with annotated masses and most of the masses are below 1000 Dalton.
• C, H, N, O, P, S are the main atoms of biological compounds.

• The possible fractional values of the compound masses are dependent on the integral compound mass.

• Mass spectrometric devices may drift, meassured m/z values can therefore vary.
• To round m/z to integral values as robust as possible, 0.75 instead of 0.5 may be used.
• In the fractional interval 0.7 to 0.75 only 15 compounds are listed in the KEGG compounds database.

• For all masses measured by a MS instrument (Ion Trap) during a complete chromatogram, the distribution of the values is comparable.
• Again, in the fractional interval 0.7 to 0.75 only 0.3% of the m/z values are detected.

• This observation is also useful if extracted ion chromatograms (EIC) of GC/MS measurements with integral m/z values need to be generated without following individual m/z traces. Again, a rounding of the m/z values at the 0.7 instead of 0.5 will provide more stable EICs.